Addresses specified in Internet Protocol version 4 (IPv4) (IPv4 addresses) have conventionally been used in communication via the Internet, and depletion of newly allocatable IPv4 addresses has recently become a problem. Accordingly, introduction of Internet Protocol version 6 (IPv6) which is allowed to allocate a greater number of addresses has been started, and various software (for example, application programs) and hardware (communication terminals such as personal computer and network relay devices such as router device) compatible with IPv6 have been proposed. The communication infrastructure includes a large number of devices which are not compatible with IPv6, so that communication terminals compatible with both IPv4 and IPv6, i.e., communication terminals having the IPv4/IPv6 dual stack structure, have been proposed as the device compatible with IPv6 (see, JP 2011-523793A and JP 2013-115639A).
Network relay devices that are connected with the communication terminal having the IPv4/IPv6 dual stack structure include a network relay device configured to selectively operate in one of an operation mode of operating as a router device and an operation mode of operating as a bridge device. This network relay device fails to normally relay an IPv4 packet without setting adequate one of these two operation modes with respect to relay of the IPv4 packet according to a connection mode (network configuration) or more specifically the presence or the absence of another network relay device on a Wide Area Network (WAN) side of the network relay device and formation or non-formation of an IPv4 over IPv6 tunnel. The operation of identifying the network configuration, the operation of selecting the adequate operation mode according to the identified network configuration and the operation of setting the selected operation mode are, however, not easy for not only unskilled users but even skilled users.